Firefox version: 87.0
OS: Arch Linux
I have been facing this problem for quite a while. Like an year or so.
I haven't been able to reliably reproduce the problem. It happens quite frequently and randomly.
Symptoms:
The buffer suddenly becomes empty, even if the buffer was fully loaded a moment ago.
Then playback starts buffering.
Then sometimes playback continues, sometimes playback restarts from beginning.
And sometimes, instead of restarting, playback resumes with video.
Other extensions installed:
If other people who are having this problem share any info they have, we could try to find what's common in all of these configurations.